The unideterminant compound L-tyrosine-p-azophenyltrimethylammonium (tyr(TMA)) can induce cellular immunity in guinea pigs with no detectable antibody produced to this immunogen. If a second identical tyr(TMA) determinant is covalently linked via a 1,6 diamino hexane spacer molecule to tyr(TMA), this symmetrical bifunctional compound, (tyr(TMA)-NH-(CH2)3)2, regularly induces anti tyr(TMA) antibody. This well-defined antigen offers the unique opportunity in that both T and B cells react and cooperate to identical linked determinants to produce antibody. It is proposed to compare the antigen recognition unit(s) on tyr(TMA) specific T and B cells by using idiotypic probes. Idiotypic antisera made against anti tyr(TMA) antibody elicited by the symmetrical byfunctional tyr(TMA) antigen will be used to examine shared V-region idiotypes. Cross reactivity of idiotypes expressed by the antigen receptors will be determined by the ability of the anti-idiotypic serum to block antigen binding, MIF production, and lymphocyte proliferation in T-cell populations. The humoral immune response to this bifunctional compound will be studied with respect to the clonal nature of antibody, classes of antibody, memory and classes of antibody associated with it. Beside its use in studying B and T cell receptors, the expected restriction in antibody production to this small antigen will be useful in analyzing the proposed "switch" of heavy chain classes in the immune response.